Abstract: From an extensive sheep flock, 20 creole mexican ewes were selected, age fluctuated between 2 and 6 years, allocated at random in groups of 10. Group 1 (n = 10) received a subcutaneous implant with 15 mg naloxone HCL. Group 2 (n = 10) was sham treated on the may 30-2008. Total 7 days after receiving the implant they were treated with GnRH (250 ng iv at 2 h interval for 24 h) and one final GnRH iv injection of 125 μg. Blood samples were collected at 12 h intervals since, the beginning of the experiment and continued until 15 days after the last injection of GnRH. In 2 of the control ewes it was observed that progesterone concentrations increased since 36 h after the first dose of GnRH until a maximal of 3 ng mL-1 of plasma progesterone concentration was reached on day 7 and the remaining 8 control ewes showed short luteal phases with a maximum of 1 ng mL-1 of plasma progesterone. In ewes treated with naloxone 3 showed short luteal phases and 7 naloxone treated ewes had plasma progesterone concentrations similar to control ewes with normal luteal phases. There was a significant effect of treatment (p<0.0019) between control and naloxone treated ewes. This results show that endogenous opioids are important modulators of reproduction in the ewe.